Various sports motions such as pitching, throwing, passing, kicking, and swinging generally involve propelling a projectile like a ball or swinging an implement such as a bat. The skill of the trainee seeking to improve such sports motions and to prevent or minimize the risk of injury generally includes training to improve body strength, balance, speed of movement, and control/coordination of movement.
For example, when pitching a baseball, the strength, control, coordination, and speed associated with the trainee's glove-side leg motion or stride, the trainee's throwing arm, and the trainee's core muscles (muscles of the torso, i.e., other than those of the arms and legs) are essential in developing pitching skill and avoiding injury.
Various sports training devices provide training for sports motions. Some devices offer elastic or weight and pulley-based resistance between an attachment point on the trainee's body and an anchor point located on a machine, wall, floor, pole, or other fixed structure. However, such off-body anchoring disturbs the trainee's balance, provides a resistance vector not adequately related to the trainee's body mechanics, and typically does not provide for the desired full range of motion or an actual release of or contact with a ball. Other sports training devices are worn by the trainee and lack off-body anchoring; however, such devices typically only provide various structures that limit or guide motion and do not offer resistance to improve core or peripheral body strength. Yet other sports training devices do provide resistance between an attachment point on the trainee's body and an anchor point on the trainee's body but fail to offer resistance that does not impede free and natural movement of the arm and/or leg, thus these devices may provide negative training of coordination of movement or even injury. Weighted balls sometimes used for such training risk overstressing and damaging muscles and joints such as the shoulder.